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J. Phys. Chem. C 2009, 113, 10830–10832
Reply to “Comment on Rapid Photoelectrochemical Method for in Situ Determination of Effective Diffusion Coefficient of Organic Compounds” William Wen, Huijun Zhao,* and Shanqing Zhang Griffith School of EnVironment, Gold Coast Campus, Griffith UniVersity, Queensland 4222, Australia ReceiVed: January 26, 2009 We appreciate L. Zhu’s comments on our recent work1 entitled “Rapid Photoelectrochemical Method for in Situ Determination of Effective Diffusion Coefficient of Organic Compounds”. We would also like to thank the editor of J. Phys. Chem. C for giving us an opportunity to present our views. We disagree with Zhu’s criticisms on our measurement principle and conclusions presented in the paper as his or her skepticisms were purely based on the experimental observations of others2-10 that were obtained under very different circumstances to serve very different purposes. We would like to note that the methodology and the conclusions presented in our work were the results of a comprehensive study based on our firsthand experimental observations. Zhu questioned our “measurement principle and most important conclusion” “because the authors used a model that neglected an important affecting factor of photocurrent, that of light intensity, as it was used for identifying the rate-limiting step and deriving effective diffusion coefficients of organic compounds”. Zhu then expressed his or her points in detail. For the readers’ convenience, we would like to give our response directly after each of the original comment made by Zhu. Original Comment “As the authors stated, only when the overall reaction is under diffusion-controlled conditions is their eq 5 valid for determination of the diffusion coefficient. This requires that the ratedetermining step in a photoelectrocatalysis system is the diffusion of organic compounds toward the surface of the electrode, and the limiting photocurrent is totally determined by the diffusion flux of solutes.” Response. We agree with Zhu’s assessment, but it should be noted that the limiting photocurrent referred to here should not be interpreted as the overall photocurrent. As clearly defined in our paper,1 the limiting photocurrent (il) we referred to was the net photocurrent originating from the photocatalytic oxidation of organic components obtained under the diffusion controlled conditions (see Figure 3 in our original paper1). The photocurrent in eq 5 should therefore be treated as the net photocurrent in accordance to the definition. We believe Zhu’s subsequent arguments (see below) were fundamentally wrong because he/she misunderstood the differences between the overall photocurrent and net photocurrent. Original Comment “The authors argued that at a given light intensity of 6.0 mW/ cm2 (corresponding to incidence photons 1.1 × 1016 s-1 cm-2 or a maximum photocurrent of 1766 µA cm-2) their system can attain diffusion-controlled conditions if the applied potential
Figure 1. Photocurrent-light intensity relationship.
bias is more positive than +0.30 V (versus Ag/AgCl) and the concentration of the substrate is below 0.8 mM for benzoic acid.” Response. Yes, for our system setup, the rate of the overall photocatalysis reactions will be under diffusion (of organics) control when 6.0 mW/cm2 light intensity (ensuring photohole generation is not the rate-limiting step) and +0.30 V versus Ag/AgCl (ensuring the effective capture of all photogenerated electrons) are employed, if the organic concentration is sufficiently low (i.e.,
